docs: Add asyncio source code links (GH-16640)

(cherry picked from commit f900064ac4b35226caad7502abc8a7e64f1c0e9d)

Co-authored-by: Kyle Stanley <aeros167@gmail.com>
diff --git a/Doc/library/asyncio-sync.rst b/Doc/library/asyncio-sync.rst
index cc8c29d..f080b03 100644
--- a/Doc/library/asyncio-sync.rst
+++ b/Doc/library/asyncio-sync.rst
@@ -6,6 +6,10 @@
 Synchronization Primitives
 ==========================
 
+**Source code:** :source:`Lib/asyncio/locks.py`
+
+-----------------------------------------------
+
 asyncio synchronization primitives are designed to be similar to
 those of the :mod:`threading` module with two important caveats: